This invention relates to an alternator voltage regulator with speed responsive control and particularly to such a regulator for an engine driven alternator.
A.C. power sources are used for various mobile and alternate power supplies. Engine driven alternators are required in various applications. The output of the alternator is regulated by a suitable electronic regulator to establish a given voltage at a given frequency. The engine driving the alternator may include a governor to establish a closed loop control of the engine speed. The regulated engine will establish the desired frequency and the electronic alternator regulator maintains the desired output voltage and current. The alternator regulators are designed to have a maximum speed of response and short time constant to changes in the output of the alternator. The alternator load effects the engine, and the effect may be significant, depending on the size of the load and the response time of the driving engine. Diesel engines, particularly such engines having a turbo-charging unit, have a relatively slow response or long-time constant in the regulator system, in comparison to that of the electronic regulators of the alternators. In the systems of the above response characteristic, a sudden increased load may result in a rapid speed drop in the engine and the driven alternator, with damaging results. In turbocharged diesel engines, the exhaust pressure runs the turbocharger. The turbocharger takes appreciable time to reach the operating speed. A sudden load application may be sufficient to create a speed drop at which the exhaust pressure decreases below that necessary to run the turbocharger up to operating speed. The result will be engine stall and complete loss of power. Similar results may occur with other engine driven system. Thus, naturally aspirated engines which are equipped with relatively slow governors may stall even with a regulator which includes a modifying fast acting "volts per hertz" ramp control to reduce the effect on the engine driving a load change which correspondingly reduces speed. For example, U.S. Pat. No. 4,219,769 which issued Aug. 26, 1980 to the present inventor and another, discloses a regulated system for alternators, with means to modify the electronic regulator for an alternator such that the alternator output voltage is reduced in a controlled manner as the frequency is reduced below a selected frequency. As a result, damage to the alternator is prevented. However, the system may not prevent engine stalling in engine driven systems wherein the rate of speed changes may be critical.
The above patent also discloses improvement in the operation of the voltage regulator by producing a response to the mean square of the alternator output which is assumed to provide the necessary constant. Alternators are rated and designed on the root mean square (R.M.S.) voltages, which is a true value of the heating effect of the alternator voltage and current.
The ratio of the RMS value of the voltage to the average value for sinusoidal waveshapes has a value of 1.11. For waves with pronounced peaks the ratio is higher and for flattened waves less than this value. In machines distortion normally gives a peak or chopped wave shape especially when load is applied and a regulator responding to the average will increase the ture RMS effect. A higher R.M.S. output may have a deleterious effect on loads such as lamps and the like. If a similar current measurement is used, an inadvertent overload may occur.
Generally, low speed operation may also result from operator misuse, an engine malfunction or fault, or running low on fuel over a protracted period and the like. Operation of the alternator under such conditions for other than short periods may result in burn-out of the alternator winding, damage to connected loads or regulator failure.
Further, certain applications may require a precise linear volts per hertz (V/Hz) error signal such that the voltage output decreases in proportion to a drop in the frequency output of the alternator. The conventional summing of the underspeed voltage signal with the sensed mean-squared voltage signal does not in fact produce the desired linear or essentially straight V/Hz curve or signal over more than a small frequency variation. For example, at 50% of rated speed, 50% of the D.C. sensing signal comes from the underspeed voltage signal and 50% from the mean square voltage signal. However, at such point, the sensed voltage is 71% of the full or rated speed voltage and when the ratio of the underspeed term is increased to give 50% of the voltage signal at the 50% speed point, the curve and output is non-linear at other points on the curve. Thus, the inventor has found that the actual R.M.S. voltage must be used and summed with linear underspeed voltage.
In summary, the prior art has not produced a satisfactory regulating system, particularly for engine driven alternator, to produce a proper modifying response to transiently or momentarily reduce the alternator load much faster than the normal V/Hz curve or characteristic so as to allow the loaded engine to recover before the load is increased to the full final value, or to produce the control with a precise V/Hz characteristic such as desirable in certain applications, for example, energizing a variable speed synchronous motor connected to drive pumps.